I know that lots of people say that John Coltrain was b. in 1738 on the Atlantic Ocean, but there is no proof of this statement.
My research is based on documented evidence. You can find the banns for John Coltrain and his wife Rebecca Muxsom published 12 Nov 1762 in the Wareham Marriages and Intentions from Town Books 1 and 2. (On the web). Rebecca, the oldest child of Edmund Muxsom and Abigail Ellis was baptized at the First Church of Wareham 10 Oct 1742 (transcription by Leonard H Smith). These transcriptions are found in my library: Denver Public Library.
I will answer your other questions off line.